When base stations are installed in an area in a cellular radio communication system within which a radio communication service is provided, the geometry of the base stations installations is determined so that each radio terminal can transmit and receive radio signals with at least one base station. A service coverage area provided by one base station is referred to as a cell. Even if a plurality of base stations are installed in the service area, a dead spot in which a radio terminal cannot transmit and receive radio signals with any of the base stations may be present due to blocking of the radio signals by a restriction of geographical features or buildings.
In a typical operation, the radio terminal, when switched on, scans preset frequencies to detect whether a connectable base station is present and issues a connection request to a detected base station to establish a connection with the base station. Transmission of radio signals from the radio terminal may be prohibited if the radio terminal fails to detect the presence of a base station. Even if the radio terminal receives a radio signal from a base station, a radio signal bearing a connection request from the radio terminal may not reach the corresponding base station. In such a case, no communication is established between the radio terminal and the base station.
A relay station may be installed to establish communication between the radio terminal and the base station in a manner free from such an inconvenience. The relay station relays a radio signal from a base station to a radio terminal while also relaying a radio signal from the radio terminal to the base station. With such a relay station, the radio terminal receives a radio signal from the relay station instead of directly receiving the radio signal from the base station, and therefore communication is consequently established between the base station and the radio terminal. The relay station also relays a radio signal that is transmitted by the radio terminal to the base station, and the base station recognizes that the relayed radio signal is transmitted from the radio terminal. Thereafter, communication is continuously established.
Specifications of the relay station have been studied by an IEEE802.16 working group under Institute of Electrical and Electronics Engineers (IEEE). According to draft specifications, a transparent mode relay station and a non-transparent relay station are defined (IEEE draft standard P802.16j/D3). In a transparent mode, the radio terminal receives control information, such as preamble, UL MAP, DL MAP broadcasted from the base station and the relay station relays data communications but does not relay the control information. In a non-transparent mode, the relay station operates as if it is equivalent to the base station with respect to the radio terminal. The relay station in non-transparent mode usually transmits the broadcast information that is normally transmitted by the base station.
Japanese Patent Laid-Open Publication No. 2006-254155 discloses a technique that allows the presence of a relay apparatus for establishing a connection to a base station without an increase in power consumption in a multihop connection between the radio terminal and the base station. According to this technique, the relay apparatus performs a predetermined reception operation for a time period that is equal to a reception slot of a control signal from the base station, thereby reducing an increase in power consumption.
If the broadcast information from the base station reaches the radio terminal in the radio communication between the radio terminal and the base station, the radio terminal transmits a connection request to the base station. If the transmitted connection request fails to reach the base station, then the base station does not recognize the radio terminal. This means that the base station does not recognize the presence of the radio terminal which requires the relay operation by the relay station. Thus, in such situation, the base station is not triggered to start the communication with the radio terminal.
If the relay station is set to be continuously ready for relay operation in order to reliably deliver the connection request from the radio terminal to the base station, the relay station continuously consumes power for the relay operation. In the situation in which a connection request may come in at an unknown timing, power is consumed in vain. In order for the relay station in the non-transparent mode to be continuously ready for relay operations, the relay station continuously transmits the broadcast information sent from the base station. The radio signal resources are used in vain if the relay station continuously transmits the broadcast information in the situation where the presence or absence of a target radio terminal is unknown. Such an operation may interfere with the surrounding communication, leading to degradation in performance of the entire radio communication system.